The invention pertains to electronic control equipment for motor vehicles and, particularly, to electronic brake control equipment for motor vehicles.
In motor vehicle wiring, undesirable potential differences often occur, due to contact resistances, especially on ground paths. This means that different types of electrical equipment present in the motor vehicle have different voltage levels with respect to ground potential. For instance, brake control equipment, such as electronic antilocking equipment or the like, can have a considerably higher ground potential than loads driven by it, such as magnetic valves, which are at ground potential on one side. This may lead to malfunctions of or damage to electronic components of the control equipment since they are being operated outside their specifications.
If the ground potential of the load is higher than the ground potential of the control equipment, the load has a higher resistance from the perspective of the control equipment, which is of no further concern for the functioning of the control equipment. In the worst case, the load will no longer work properly; for instance, a solenoid will not activate, despite applied exciter potential. In contrast, if the ground potential of the load is lower than the ground potential of the control equipment, then the ground potential of the load will appear as a negative potential from the potential of the control equipment. This can lead to overloads on the control equipment and to the destruction of some components in it.
To solve this problem it has thus far only been possible to pull a special ground line through the vehicle, which represents a considerably increased expense in wiring, but does not exclude the possibility of potential differences because of contact resistance at plug connections.
German patent reference DE 42 10797 discloses a circuit for monitoring an electric line for short circuits or broken lines. As shown in the reference, a speed circuit, which is connected between positive supply voltage and ground potential, provides electrical pulses representing the speed of a vehicle. When a first transistor is switched on, the potential of one connector will be the ground potential; when the first transistor is switched off, this potential will be the positive supply voltage level. This known circuit has two test circuits. The first test circuit, which includes a first resistor, is always active and checks for a broken line. In this case, a second resistor is no longer connected with the input of a trigger circuit and, therefore, the first resistor always connects the positive supply voltage to the input of the trigger circuit. In this state, the trigger circuit and associated microprocessor do not receive pulses but only a constant voltage which is recognized by the microprocessor. However, in this case a test current does not flow through the speed circuit.
A second test circuit disclosed by the German patent reference DE 42 10797 includes the trigger circuit and microprocessor. This second test circuit is active when the pulse-like signals from the speed circuit have a very low frequency. When the microprocessor detects this state, it gives a pulse on its output to the second test circuit. A pair of transistors as well as a diode are switched on and, via a third resistor, an electrical pulse is fed to the input of the trigger circuit. If the speed circuit or line do not have a short circuit, this pulse switches the trigger circuit on, which is detected by the microprocessor. In case of a short circuit of the line, the test pulse is suppressed and the trigger circuit does not switch. This is detected by the microprocessor as a fault. In this case, a test current pulse is used but the magnitude of this current is not measured. The second test circuit only checks whether the voltage on the input to the trigger circuit has a threshold value for switching the trigger circuit.
Thus, it is not possible to detect a difference between the ground potential of a load and the ground potential of a control apparatus with the circuit disclosed in DE 42 10797. Even if the ground potential of the speed circuit at the emitter of the first transistor is different with respect to the ground potential of the control apparatus, the difference cannot be detected by the test current pulse because this test current pulse is produced just at a time when the first transistor is switched off. When the first transistor is switched off, the ground potential on its emitter has no influence on the ground potential of its collector. Therefore, the trigger circuit would detect a correct test pulse and the circuit would not detect an error.
Therefore, a control device is needed which advantageously causes a test current to flow through a load in response to a ground potential difference, measures the magnitude of the test current and initiates countermeasures to protect the control device from damage resulting from differences in the ground potentials. Further, a control device is needed which produces the test current only when a difference in the ground potentials occurs. This is particularly beneficial because in monitoring an electronic circuit, monitoring circuitry generally must be active in order to detect an error.